What are Basic or Cationic DYES?
A basic dye is a cationic dye distinguished by its formability for acidic types of acrylic fibre and tannin-mordant cotton, reflecting the former importance of this class of dye in cotton dyeing, for which reactive dyes, direct dyes, vat dyes, and sulphur dyes are now used. When basic dyes are dissolved in water, they form colored cations with characteristically high durability strength and brilliance. These dyes are referred to as cationic dyes.
Perkin’s Mauve, the first synthetic dye, belongs to the basic or cationic dye class. Magenta and Malachite Green are two other early synthetic dyes. The dyes are called basic because they are made from organic bases. They are also known as cationic dyes because they ionise in water and produce colored cations. There are different kinds of dyes that are used as per the requirement of the product such as solvent dyes, organic dyes, acid dyes, synthetic dyes and basic dyes. Let’s read more about this.
What are Cationic Dyes?
For PAN fibres, cationic dyes that are used are of two types – localised dyes (aka pendant dyes) in which the positive charge is localised on one atom (e.g. N) as exemplified by C.I. Basic Orange 30:1 and C.I. Basic Blue 22, and delocalised dyes in which the positive charge is delocalised over the entire dye molecules as exemplified by C.I. Basic Yellow 28 and C.I. Basic Blue 41.
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Manufacture of Basic Dyes
- Auromine O: An essential basic yellow, this is highly valued for the exceptionally pure tints it produces. It is produced by heating to 175°C a mixture of 4,4′-dimethylamino diphenyl methane, sulphur, ammonium chloride, and sodium chloride (acting as diluents) in the presence of gaseous ammonia. The resulting Auromine base is converted into its hydrochloride, Auromine O.
- Malachite Green: It is made by combining two molecules of dimethylaniline with one molecule of benzaldehyde. The resulting leuco base is then oxidised with oxidising agents with high electrochemical potential, such as lead dioxide, to yield the dye salt with acid.
- Methylene Blue: It is produced by oxidising an equimolar mixture of dimethylaniline, p-amino-dimethyl aniline, and sodium thiosulphate with sodium dichromate and hydrochloric acid in the presence of zinc chloride. This is done to get the final dye as zinc chloride double salt.
- New Magenta: In an enamelled vessel, 1 kg of anhydrous formaldehyde, 5 kg of toluidine hydrochloride, and 1 kg of o-toluidine are heated for 2-3 hours at 170°C with 1.2 kg of o-nitrotoluine and 100 grams of iron fillings. The unreacted o-toluidine and o-nitrotoluene are removed through steam distillation, the residue is hot-filtered, and the New Fuscsin is salted out.
Features if Basic Dyes
Here are some of the vital characteristics of basic dyes:
- Ionic nature: By nature, these dyes are cationic.
- Shade range: These dyes have an infinite colour range, high tinctorial strength, brightness, and many colours have fluorescent properties.
- Solubility: These dyes are very soluble in water in the presence of glacial acetic acid.
- Levelling properties: Since these dyes have a high strike rate, levelling is inferior.
- Exhaustion: Cationic dyes exhaust at varying rates; K values are used to define the cationic dyes’ exhaustion characteristics. K=1 represents the quickest exhaustion, while K=5 represents the slowest. As a result, dyes with equivalent K values must be used when creating the combination shades.
- Affinity: These dyes have a high affinity for wool, silk, and cationic dye-able acrylic, but no affinity for cellulosic. To dye cellulosic with basic dyes, the material must first be mordant.
- Fastness Properties: The light fastness ranges from poor to moderate, but the wet fastness is excellent of these basic dyes.
Given below are some of the other functions:
- Can be applied to anionic substrates in areas where electrostatic attractions form
- Relatively economical in usage
- Functions as potent coloring agents
- Can provide a variety of shades with high tinctorial strength and varied brightness.
- Can be used on silk, wool, cotton, and modified acrylic fibres
- Also used in coloration of paper
- They function as cationic soluble salts of coloured bases.
- In the presence of glacial acetic acid, it offers high solubility in water